1. Field of the Invention
The present invention relates generally to packet communication, and more specifically to a method and an apparatus for scheduling packets for downlink transmission to mobile terminals from a cell-site base station.
2. Description of the Related Art
A number of packet scheduling techniques are known in the art. One of the known techniques is the round-robin system which is currently under study by the High Speed Downlink Speed Access System (HSDPA) of the 3rd Generation Partnership Project (3GPP) and is known as “Physical Layer Aspects for High Speed Downlink Packet Access”, 3GPP TR25. 848. 2001. In this system, downlink packets are transmitted in a round-robin fashion from a base station equally to mobile terminals. Since fairness is ensured when assigning time slots, equitable throughputs are realized. Another technique known as the Max C/I system (“Physical Layer Aspects for High Speed Downlink Packet Access”, 3GPP TR25. 848. 2001), also currently under study by 3GPP, uses the carrier-to-interference ratios of wireless links and transmits downlink packets in a descending order of their C/I ratios. However, these known techniques are still unsatisfactory in terms of fairness and throughput. Additionally, in both of these known techniques no consideration is taken with regard to quality of service.
According to another technique called “CBQ with CSDPS” as discussed in a paper “Controlled Multimedia Wireless Link Sharing via Enhanced Class-Based Queuing with Channel-State-Dependent Packet Scheduling”, Fragouli et al., INFOCOM '98, packets are transmitted at guaranteed data rates according to different services and residual bandwidths are equally assigned to mobile terminals. Qualities of wireless links are also taken into account for downlink transmission. However, packet transmission is perform in so far as the wireless link satisfies a predetermined quality level. As a result, the data rates of all downlink packet transmissions of a whole cell-site tends to decrease to a low level.